Add a possibility to refresh a binding from the code-behind.

[VladiStep]

Is your feature request related to a problem? Please describe.

In WPF, it's possible to do something like this: textBlock.GetBindingExpression(TextBlock.TextProperty).UpdateSource().
But it seems you can't do that in AvaloniaUI.

Describe the solution you'd like

There should be some alternative to the WPF method I described.

Describe alternatives you've considered

I guess it's needed to rework an existing internal BindingExpression class so that you can get it for a property and then call UpdateSource() on it.

[maxkatz6]

There is no UpdateSourceTrigger=Explicit support yet, and this API doesn't make sense without it.
But it is planned bee98ad#diff-ccfdf6768edfce7a5cf7bb4a103e15abeeb53ba684bf457df6d52d3f61a9cdb2

[timunie]

Avalonia.Behviors may be useful for the time beeing. See: #3754 (comment)

[grokys]

I'm currently working in a binding refactor which includes this feature. It's not quite ready for a PR yet but the brach is refactor/bindings.